1. Field of the Invention
The present invention relates to an objective lens driving apparatus for adjusting position of beam irradiation by moving the objective lens in an optical pickup apparatus used in an optical information recording and reproducing apparatus and the like.
2. Description of the Background Art
An optical pickup apparatus is structured such that a light beam is converged on a recording surface of an optical disk through an objective lens. The objective lens is mounted on an objective lens driving apparatus. The position of the objective lens is controlled by the objective lens driving apparatus in accordance with the detected amount of change of the light reflected from the optical disk so that the convergent light of the beam sequentially traces accurately the prescribed track on the recording surface of the optical disk. By the objective lens driving apparatus, the objective lens is driven in two directions, that is, the optical direction of the objective lens or a focusing direction perpendicular to the recording surface of the optical disk, and a direction orthogonal to the optical axis, for example, the tracking direction parallel to the recording surface of the optical disk.
In a conventional axially sliding type objective lens driving apparatus, an axis is fixed vertical to the base. This axis is the central axis for rotation as well as the central axis for sliding. A guide cylinder of the objective lens is fitted to the axis. The objective lens is attached to the objective lens holder. The objective lens holder is movably attached on the base with an elastic member interposed. Thus, the objective lens holder can rotate and slide in the transformable range of the elastic member. A focusing coil is wound around the outer periphery of the objective lens holder. In addition, a tracking coil is provided on the focusing coil. A pair of permanent magnets are provided near the outer peripheral portion of the objective lens holder so as to oppose to each of the coils. The permanent magnets are attached to an inner yoke portion and an outer yoke portion. The inner yoke portion and the outer yoke portion are fixed at the base.
In a different type objective lens driving apparatus, the objective lens holder is supported on the base by supporting means having a link mechanism. The support means includes a metal spring or a resin molded body. A pair of permanent magnets are fixed on the base. A focusing coil is mounted on the objective lens holder opposing to the permanent magnets. The focusing coil is provided to surround the inner yoke portion. A tracking coil is provided over the focusing coil.
In the above described objective lens driving apparatus, the narrower the gap between the permanent magnet and the inner yoke portion becomes, the stronger the magnetic field generated in the gap becomes. Therefore, even if the control signal current flowing through the focusing coil and through the tracking coil is relatively small, a prescribed power can be obtained.
However, since the tracking coil is provided overlapped on the focusing coil, there is a limit in narrowing the gap mentioned above. Further, since the electromagnetic force of each coil influences with each other to decrease sensitivity in driving in the respective directions, the problem of crosstalk occurs.
In order to solve the above described problem, an objective lens driving apparatus is proposed in Japanese Patent Laying-Open No. 64-8518. A radial anisotropic magnet is provided in the objective lens driving apparatus. The radial anisotropic magnet has a magnetizing vector in a direction perpendicular to the cylindrical surfaces of the objective lens holder. An inner yoke portion and an outer yoke portion are formed to sandwich the cylindrical surface of the objective lens holder. A focusing coil and a tracking coil are mounted on cylindrical surfaces opposing to the outer yoke portion and the magnet. Thus a magnetic circuit is formed.
In order to solve the above-described problem, another objective lens driving apparatus is proposed in Japanese Patent Laying-Open No. 64-14736. In this objective lens driving apparatus, the focusing coil and the tracking coil are formed not to overlap with each other in the radial direction of the objective lens holder. Thus the magnetic circuit is formed such that the magnetic field traversing each coil is formed opposite to each other.
In the meantime, down sizing, reduction in weight, low power consumption and low manufacturing cost of product such as optical disk apparatus and so on containing the optical pickup apparatus have been desired. Therefore, down sizing, reduction in weight and lower manufacturing cost are also desired for the objective lens driving apparatus incorporated in the optical pickup apparatus.
However, in order to further reduce the size of the conventional objective lens driving apparatus, the area on which the plurality of focusing coils and tracking coils attached on the objective lens holder are to be soldered must be reduced. This causes decreased efficiency in the manufacturing steps of the objective lens driving apparatus. If the objective lens driving apparatus having the conventional structure is further reduced in size, it leads to increase manufacturing cost and it may cause degradation in quality because of, for example, defective soldering. As a method for solving these problems, permanent magnet may be provided on the side of the objective lens holder in the objective lens driving apparatus. This makes it unnecessary to attach the coils on the objective lens holder. Therefore, power supply to the movable objective lens holder becomes unnecessary, and soldering in a small space also becomes unnecessary. However, since the permanent magnet is provided on the movable objective lens holder it becomes difficult to form a magnetic circuit including the yoke portion.
Accordingly, one objective lens driving apparatus is disclosed in Japanese Patent Laying-Open No. 60-191441. In this objective lens driving apparatus, the permanent magnet is provided on a movable body holding the objective lens. A coil for adjusting focusing position and the tracking position is provided on the support body opposing to the permanent magnet. In this objective lens driving apparatus, the movable body can move in the tracking direction and in the focusing direction without any yoke portion. However, in this objective lens driving apparatus, the driving power of the objective lens holder (movable body) is decreased because of the influence of the weight of the permanent magnet. This results in degraded following characteristic of the objective lens by a servo circuit, which characteristic is important in the performance of the optical pickup apparatus. Further, in some products such as optical disk apparatuses using batteries as power supply, power consumption is increased.
Therefore, the technical problem to be solved is how to make smaller the gap between the permanent magnet and each coil and to reduce as much as possible, the number of parts which increases the weight of the balancer and the like for the objective lens.